1. Field of the Invention
The present invention relates to a method and apparatus for adjusting the black level and white level of the luminance component of a digitized picture signal in a television receiver or the like.
2. Conventional Art
In a television picture with few black areas, the black areas tend to stand out from their surroundings. This is undesirable. To prevent this effect, it is known to adjust the black level as shown in FIG. 12. In FIG. 12, the horizontal axis represents the input luminance signal, and the vertical axis represents the output luminance signal. The straight line indicates the input-output characteristics without adjustment, and curves indicate input-output characteristics with adjustment. By the adjustment of the input luminance signal as shown in the range below 50 [IRE] of the input, the difference between the darkest part and the adjacent part (which is a little brighter than the darkest part) becomes smaller, so as to prevent the darkest part from standing out. This adjustment is also called stretching since the black parts (except the part of the lowest brightness) are moved farther away from the central value (50 [IRE]). Incidentally, the letters IRE indicate the amplitude scale standardized by the Institute of Radio Engineers.
By making similar adjustment in the range above 50 [IRE of the input, white level adjustment can be achieved.
To realize the non-linear characteristics shown by FIG. 12, ROMs are often used. However, the capacity of the ROMs tend to be very large.